1. Field of the Invention
The present invention relates to a double-walled housing, particularly for exhaust catalytic converters of motor vehicles which include a metal catalyst carrier through which exhaust gas can flow. The present invention also relates to a method for producing a double-walled housing.
Many forms of catalyst carriers are known. One example of a metal catalyst carrier can be learned from Published European Patent Application 0 245 737 A1, corresponding to U.S. Pat. No. 4,832,998. That catalytic converter body can, for example, be built up from metal sheets or produced by powder metallurgy.
Such catalyst carriers are disposed in a housing. Due to the mechanical strains in such a body that arise from alternating thermal stresses, proposals have already been made for how such strains might be reduced by suitably shaping or especially constructing the sheathing.
European Patent 0 435 956 B, corresponding to U.S. Pat. Nos. 5,173,267 and 5,190,732, discloses a double-walled housing for exhaust catalytic converters, especially for motor vehicles. The honeycomb-like catalyst carrier is supported by an inner jacket tube that is disposed in an outer jacket tube. The inner jacket tube is disposed in such a way as to be longitudinally expandable in the outer jacket tube. The free longitudinal expandability is attained through the use of a sliding fit.
German Utility Model DE 74 01 002 U discloses a housing for a catalyst carrier body that has an inner jacket and an outer jacket. That housing serves to receive a ceramic catalyst carrier. A compressible porous damping layer is disposed between the inner jacket and the catalyst carrier.
In order to meet ever more stringent environmental laws, the cold starting and restarting properties of exhaust gas catalytic converters are becoming increasingly more important. The thermal insulation properties of housings play an important role in those properties, so that improvements are sought while maintaining the same economy of manufacture.
It is accordingly an object of the invention to provide a double-walled housing, in particular a sheathing for exhaust gas catalytic converters of motor vehicles, and a method of producing a double-walled housing, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type, which can be manufactured simply and economically and which has great stability and favorable thermal insulation properties. The housing should also be suitable for other portions, for instance conical portions, of an exhaust system, particularly with a view to its thermal insulation properties.
With the foregoing and other objects in view there is provided, in accordance with the invention, a double-walled housing, in particular for exhaust gas catalytic converters of motor vehicles, comprising an inner jacket having two end regions; an outer jacket approximately concentrically surrounding the inner jacket at a spacing, the outer jacket having constrictions keeping the inner jacket under compressive strain at least at the two end regions, and the outer jacket having embossed features oriented toward the inner jacket; and the inner jacket and the outer jacket having end surfaces resting tightly against one another.
In accordance with another feature of the invention, the embossed features are distributed approximately uniformly over the length of the housing and the circumference, in order to attain uniform mechanical stability.
The jackets may be formed of metal sheets having a total wall thickness which is less than the thickness of the single housing wall known previously. In order to obtain adequately great stability of the housing, the outer jacket is provided with embossed features pointing toward the inner jacket that are preferably formed symmetrically on the outer jacket. These embossed features are preferably constructed in such a way that they lend the housing great stability in both a radial and a tangential direction. In this way and because of the interstice formed between the two jackets that acts as a thermal insulation, the inner jacket tube can heat up faster with the catalyst carrier and overall can reach a higher temperature than without the outer jacket. The thermal stresses in the interior of the catalyst carrier are reduced due to this property. Moreover, the inner jacket is not exposed to any external influences, so that there is a wider spectrum of materials from which a choice can be made. The inner jacket can also be reduced in terms of its wall thickness due to these properties of the housing.
In accordance with a further feature of the invention, the thermal insulation properties of the housing can also be purposefully adjusted by provided that some of the embossed features in the outer jacket touch the inner jacket, or that additional insulating layers or coatings are provided on or between the two jackets. Besides heat conduction, which is best reduced through the use of a gap between the two jackets, convection, which is best prevented by a ceramic intermediate layer, plays a role. However, another factor that is important at higher temperature is heat radiation, which can be reduced or reflected through the use of suitable coatings. The housing according to the invention offers the possibility of purposefully adapting these properties to demands made of them, especially in catalytic converters. As a result, the cold-starting and re-starting properties of such a catalytic converter can be improved.
In accordance with an added feature of the invention, the outer jacket tube includes a round or conically shaped, preferably rolled, metal sheet, and the abutting edges are joined together by joining techniques. The term xe2x80x9cround-rolledxe2x80x9d is understood to mean other than round-cross-sectional forms as well and may, for instance, involve oval or elliptical forms.
In accordance with an additional feature of the invention, the inner jacket includes a round-rolled metal sheet, with a parting line extending over the entire length of the jacket. This feature has the advantage of simplifying manufacture, because the inner jacket, as viewed in its circumferential direction, is not closed and therefore one less seam needs to be made.
In accordance another feature of the invention, the joining seam is a laser-welded seam.
In accordance with yet another feature of the invention, the inner jacket and the outer jacket, respectively, have a wall thickness of from 0.3 to 1 mm.
In accordance with a further feature of the invention, the inner jacket and the outer jacket have a maximum spacing therebetween of approximately 0.3 to 4 mm and preferably a maximum spacing of approximately 0.5 to 1 mm.
With the objects of the invention in view there is also provided a method for producing a double-walled housing, in particular for exhaust gas catalytic converters of motor vehicles, which comprises forming an inner jacket by deforming, preferably rolling, a metal sheet into cylindrical or conical form while maintaining a parting line; forming an outer jacket by deforming, preferably rolling, a metal sheet into cylindrical or conical form and connecting mutually meeting edges of the metal sheet by a joining technique; placing the inner jacket approximately concentrically in the outer jacket at a spacing with the inner jacket and the outer jacket resting tightly against one another on two end surfaces; plastically deforming the outer jacket inwardly on both end surfaces over its entire circumference for keeping the inner jacket under compressive strain; and providing the outer jacket with embossed features oriented toward the inner jacket beforehand or simultaneously with the deforming step.
In accordance with another mode of the invention, an insulating layer is placed between the inner jacket and the outer jacket prior to the deformation of the outer jacket.
In accordance with a further mode of the invention, the inner jacket tube is also provided on its outer surface and/or the outer jacket tube is provided on its inner surface with a coating, for instance an oxide film, that influences radiation properties.
In accordance with a concomitant feature of the invention, in a subsequent installation, the end surfaces of the inner jacket tube and the outer jacket tube are welded together before or during joining to an adjacent part.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a double-walled housing, in particular for exhaust gas catalytic converters of motor vehicles, and a method of producing a double-walled housing, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.